1. Field of the Invention
The present invention relates to a method of and apparatus for inspecting the surface irregularities of a transparent plate such as a plate of glass and a plate of resin, particularly a glass plate for use in a liquid crystal display.
2. Description of the Related Art
A glass sheet or plate made by the known float process has an excellent surface smoothness and flatness so it is used widely other than being used as window glass for automotive vehicles and buildings. Particularly, a glass plate for use in a liquid crystal display is required to be small in the surface irregularities, so it is necessitated to be ground further after being made or formed by the float process.
For inspection of the surface irregularities of such a glass plate, it is frequently used a method of irradiating a beam of light onto a surface of a glass plate and projecting a reflected beam of light from the surface of the glass plate on a screen for inspection, as disclosed in Japanese Patent Provisional Publication Nos. 1-212338, 5-272949 and 7-128032.
Japanese Patent Provisional Publication No. 1-212338 discloses an apparatus for measuring the surface irregularities of an ordinary glass plate wherein a beam of light is irradiated onto the glass plate at an angle of incidence ranging from 70 to 85 degrees. In this instance, the term xe2x80x9cangle of incidencexe2x80x9d indicates the angle formed by a beam of light arriving at a surface and the perpendicular to that surface at the point of arrival.
Japanese Patent Provisional Publication No. 5-272949 discloses a method of evaluating the properties of a glass plate required to have a high flatness or eveness for use in a liquid crystal display. In this method, an optical multi-line surface irregularity measuring apparatus including an optical displacement meter made up of a laser beam source and a line sensor is used for measuring the surface irregularities of the ground surfaces of the glass plate.
The Japanese Patent Provisional Publication No. 7-128032 discloses a method of inspecting the waviness of the surfaces of a glass plate for a liquid crystal display wherein an ultraviolet ray is irradiated onto a glass plate in order to eliminate reflection from the rear surface of the glass plate.
In this instance, the rear surface reflection means, as shown in FIG. 7, that the beam of light incident onto the plane of incidence (front surface) is transmitted through the transparent body and reflected by the rear surface to go back to the plane of incidence again. Thus, if there is a beam of light reflected by the rear surface, the reflected light from the transparent body contains not only the information on the surface irregularities of the front surface but the information on the surface irregularities of the rear surface, thus causing a problem that accurate information on the surface irregularities of the front surface cannot be attained.
For example, a problem of the measurement device disclosed by Japanese Patent provisional Publication No. 1-212338 is that since the beam of light is irradiated onto the glass sheet at an angle of incidence ranging from 70 to 85 degrees it is incapable of avoiding the influence of the reflected beam of light from the rear surface and therefore accurate measurement of the surface irregularities cannot be attained.
The method disclosed by Japanese Patent Provisional Publication No. 5-272949 considers nothing about the reflection by the rear surface of the glass plate, thus causing a problem that an erroneous measurement due to reflection of a laser beam from the rear surface of the glass plate is unavoidable and furthermore use of a laser beam inevitably leads to an expensive cost of the device.
Further, the method disclosed by Japanese Patent Provisional Publication No. 7-128032, though it can eliminate reflection of light from the rear surface of the glass plate, requires a light source of particular short-wavelength ultraviolet rays and furthermore a screen to which a fluorescent material is applied, thus causing a problem that a costly device is necessitated and further an operator is needed to take care in handing the device so that his eyes and skin are not damaged by ultraviolet rays irradiated.
It is accordingly an object of the present invention to provide a method of detecting front surface irregularities of a glass plate which enables to attain accurate detection by preventing rear surface reflection by the use of a light source and screen which are not of the particular kinds but usually used.
It is a further object of the present invention to provide an apparatus for carrying out the method of the foregoing character.
To achieve the above object, the present invention provides a method of inspecting surface irregularities of a transparent plate comprising irradiating a beam of light from a light source toward a surface of the transparent plate at an angle of incidence ranging from 86 to 89 degrees, projecting a reflected image of the beam from the surface of the transparent plate on a screen, and inspecting irregularities of the surface of the transparent plate on the basis of the reflected image projected on the screen.
According to another aspect of the present invention, there is provided a method of inspecting surface irregularities of a transparent plate comprising irradiating a beam of light from a light source toward a surface of the transparent plate at an angle of incidence ranging from 60 to 89 degrees, polarizing the beam of light from the light source by a polarizing element disposed between the light source and the transparent plate so that the beam of light from the light source is incident on the surface of the transparent plate as one of a P-polarized light beam and an S-polarized light beam, projecting a reflected image of the beam from the surface of the transparent plate on a screen, and inspecting irregularities of the surface of the transparent plate on the basis of the reflected image projected on the screen.
In the above described method of inspecting surface irregularities of a transparent plate, the angle of incidence of the S-polarized light beam and the angle of incidence of the P-polarized light beam are in the range from:
85 to 89 degrees and 87 to 89 degrees when a visible ray transmittance of the glass plate is 90% or more and less than 100%, respectively;
84 to 89 degrees and 87 to 89 degrees when the visible ray transmittance is 80% or more and less than 90%, respectively;
83 to 89 degrees and 87 to 89 degrees when the visible ray transmittance is 70% or more and less than 80%, respectively;
81 to 89 degrees and 86 to 89 degrees when the visible ray transmittance is 60% or more and less than 70%, respectively;
78 to 89 degrees and 84 to 89 degrees when the visible ray transmittance is 50% or more and less than 60%, respectively;
72 to 89 degrees and 82 to 89 degrees when the visible ray transmittance is 40% or more and less than 50%, respectively;
60 to 89 degrees and 75 to 89 degrees when the visible ray transmittance is 30% or more and less than 40%, respectively; and
60 to 89 degrees and 60 to 89 degrees when the visible ray transmittance is less than 30%, respectively.
According to a further aspect of the present invention, there is provided an apparatus for inspecting surface irregularities of a transparent plate comprising a light source irradiating a beam of light toward a surface of the transparent plate at an angle of incidence ranging from 86 to 89 degrees, and a screen on which a reflected image of the beam of light from the surface of the transparent plate is projected.
According to a still further aspect of the present invention, there is provided an apparatus for inspecting surface irregularities of a transparent plate comprising a light source irradiating a beam of light toward a surface of the transparent plate at an angle of incidence ranging from 60 to 89 degrees, a polarizing element capable of adjusting a polarizing angle, disposed between the light source and the transparent plate so that the beam of light from the light source is incident on the surface of the transparent plate as a polarized light beam, a screen on which a reflected image of the polarized light beam from the surface of the transparent plate is projected, and inspecting means for inspecting the surface irregularities of the transparent plate on the basis of the reflected image projected on the screen.